ITMI952141A1 - PICKLING AND PASSIVATION PROCESS OF STAINLESS STEEL WITHOUT THE USE OF NITRIC ACID - Google Patents

Abstract

Pickling procedure of stainless steel consisting of putting the material to be treated in contact with a bath maintained at a temperature between 30 ° C and 70 ° C and having the following initial composition: a) H2SO4b) FE 3 + c) HF d) emulsifying additives, wetting agents, brighteners and acid attack inhibitors; in said bath being continuously fed: - an air flow of at least 3m3 / h per m3 of bath, and a quantity regulated according to the Redox potential of the bath which must be maintained at a value> 250 mV.

Description

Description of the patent application for Industrial Invention entitled: "Process of pickling and passivation of stainless steel without the use of nitric acid"

DESCRIPTION

Technical problem

During the manufacturing processes of steel products subjected to hot rolling operations or intermediate products that undergo heat treatments such as, for example, annealing, it has always been known that the material is covered with a more or less thick layer of products of oxidation. Therefore, taking into account the need to obtain a degree of glossy and brilliant surface finish of the finished products, it is necessary to remove all of these oxide layers. This is carried out through the well-known pickling processes, for which mixtures of inorganic mineral acids are generally used, such as hydrochloric, sulfuric, nitric and hydrofluoric, alone or in various ratios.

In the field of stainless steels, based on the knowledge of the industrial processes in use today, the pickling operation most commonly, or almost exclusively, used involves the use of a mixture of nitric acid and hydrofluoric acid, whose reciprocal concentrations vary depending on the type of plant, the type of steel to be pickled, its surface characteristics and the geometry of the product to be treated. The process is certainly economical and allows to obtain excellent results; however, it has the very serious drawback of creating very significant ecological problems that are difficult to solve due to the use of nitric acid. In fact, on the one hand, nitrogen oxide vapors of the general formula ΝOx are emitted into the atmosphere, highly polluting and aggressive towards the metallic and non-metallic materials with which they come into contact, on the other hand in the washing water and in the exhausted baths high levels of nitrates are reached, with consequent subsequent need for disposal. 1 purification problems of both the ΝOx in the air and the nitrates in the baths entail major plant problems, high operating costs and the unsafe achievement of the results required by current regulations; in the final analysis, therefore, the burden in terms of investments is difficult to sustain in most industrial plants.

A pickling system that does not require the use of nitric acid is therefore required by the industry and various proposals, especially in the last ten years, have been made around the world.

Alternative procedures to the use of nitric acid: state of the art

A critical examination of the patents relating to cycles proposed as an alternative to the traditional HNΟ3 + HF-based stainless steel pickling process, which no longer contain nitric acid, and of the main technical literature on the subject has ascertained the following:

A) the two Japanese patents JP 55018552 published on 8.2.80 - and JP 55050468 published on 12.4.80 - foresee three phases:

(1) an initial descaling in sulfuric or hydrochloric acid,

(2) a subsequent immersion, the first, in a solution of potassium permanganate and inorganic acids (not HF) and, the second, in a solution of ferric nitrate, ferric sulphate and peroxidisulphuric acid,

(3) a final washing with pressurized water or ultrasound B) the Swedish patent SE 8001911 published on 12.10.81 - refers to a treatment, to be carried out for times between 1 and 120 minutes (1-20 'recommended), and at a temperature between 10 and 90 ° C (recommended 30-60 ° C), in a solution consisting of sulfuric acid and hydrogen peroxide

C) German patent DE 3937438 published on 30.8.90 - is aimed above all at the yarn treatment industry and involves the use of a hydrofluoric acid solution containing Fe <3+> added as an additive in the form of complex fluoride ; then, a gas and / or an oxygenated fluid medium is also introduced into the solution, which is subjected to an electrolysis process to obtain nascent oxygen capable of oxidizing the iron from divalent to trivalent

D) the English patent 2,000,196 of Τ D K provides for the use of a pickling bath consisting of ferric sulphate and hydrofluoric acid: to maintain an adequate concentration of ferric ions during the process, H2SO4 and hydrogen peroxide are continuously fed in a molar ratio 1: 1. The method of control of the procedure is claimed by continuous detection of the Redox potential of the system which must be maintained at values ≥ 300 mV by regulating the intake of H2SO4 + H2O2.

E) the two very similar European patents EP 188975 and EP 236354 (= W0 87/01739) respectively with priority date 22.1.85 and priority date 19-9-85, provide for the use of a pickling solution consisting of hydrofluoric acid (5-50 g / l) and trivalent iron ion introduced in the form of fluorinated complexes, into which air or oxygen is continuously blown; the treatment time is from 30 "to 5 ', the temperature varies from 10 ° to 70 ° C; a continuous control of the Redox potential is also recommended, which must be maintained, for the first patent, between -200 and 800 mV and, for the second, between 100 and 300 mV and a possible addition, if necessary to raise the potential value, of an oxidant such as potassium permanganate or hydrogen peroxide is suggested. All the tests performed are related to the pickling of Finally, there are two other patents that refer to the possibility of avoiding or minimizing the formation of nitrogen oxides ΝOx in baths that work with nitric acid, by directly introducing suitable oxidants into the pickling bath: first, Japanese JP 58110682 of .7.83 - foresees the use of hydrogen peroxide; the other, Swedish (SE 8305648 of 15-4.85 - priority date 14.10.83, SE 835648), always foresees the use hydrogen peroxide and / or, alternatively, urea.

Despite this proliferation of patents, however, to date, the traditional process based on nitric acid and hydrofluoric acid is still widely used all over the world and none of the alternatives proposed so far, and previously briefly described, have had an industrial follow-up.

A substantial progress has been achieved with the pickling process described in the European patent 562.121 of the applicant which uses a bath of H2SO4 + HF containing Fe <3+> and Fe <2+> ions operating at controlled Redox potential in which the reoxidation of the ion Fe <2+> with ferric ions is obtained by periodic additions of H2O2 to the pickling solution.

Process according to the invention

The process object of the present patent application constitutes a technically valid and in some respects advantageous alternative from the economic point of view of the known processes mentioned above in particular it constitutes a valid integration of the previous patent EP 582.121 of the applicant.

The process is based on the use of a pickling bath containing iron, HF, HCl ions and conventional additives such as wetting agents, emulsifiers, brighteners, inhibiting agents, into which it is introduced continuously or periodically

an oxidizing agent capable of converting Fe <2+> which is formed in the pickling process into Fe <3+> maintaining the Redox potential of the pickling solution at the predetermined value. The oxidizing agent can be selected from the following classes of compounds: a) peroxygenated acids such as persulfuric acid, and its salts b) oxygenated chlorine acids: chlorine and chloric in the form of their alkaline salts such as Na ClO2, Na ClO3

c) soluble permanganates.

All the aforesaid oxidants can be introduced into the bath as such or in the form of an aqueous solution.

It generally operates at a temperature between 30 ° C and 70 ° C: the temperature largely depends on the type of steel and the type of plant for which the possibility of using mechanical treatments upstream of the chemical pickling process is fundamental. descaling. The fundamental characteristic factors of the process are illustrated below.

It is very important to carry out an efficient stirring of the pickling bath in order to continuously renew the pickling solution in contact with the metal surface to be treated. A very effective method is to continuously blow a strong air flow into the bath.

Content of inorganic mineral acids in the bathroom

The functions of both hydrofluoric and hydrochloric acids are varied: among the most important are to maintain the pH of the process at values below 2.5 and to remove the oxides from the heat treatments from the metal surface. Hydrofluoric acid has the task of complexing the Fe <3+> and Cr <3+> ions in the greatest possible quantity and of depassivating the obscured material, bringing its electrode potential to an active and / or active / passive dissolution zone. (see below): in the absence of hydrofluoric acid the working potential rises in the stable passivity range of the material and descaling practically does not take place. Hydrochloric acid contributes to the total and free acidity of the solution, and to the dissolution of the oxide layers. Since, during the pickling process, the contents of the two acids, and especially that of the hydrofluoric acid, tend to decrease, it is necessary to periodically feed them according to the analytical results of the bath (values of free acidity and fluoride ions) . The concentration of the acids in steady state conditions varies, depending on the treated material, between 5 and 50 g / l for hydrofluoric acid and between 20 and 100 g / l for hydrochloric acid. The pH measured in the pickling solution as it is (without dilution) with Crison 2002 neutral pH with "ingold" electrode is generally lower than 2.5 at room temperature.

Content in the Fe <3+> ion bath: the pickling solution contains a quantity of Fe <3+> ions not less than 15 g / l and preferably at least 30 g / l, initially introduced in the form of ferric sulphate or chloride: the the function of this ion is to replace nitric acid as an oxidizing agent according to the reaction 2Fe <3+> + Fe -> 3Fe <2+>, favored by the pH conditions of the bath. During the process it is necessary to continuously create the conditions for the iron dissolved in the bath to be partially present in the form of Fe <3+>. The oxidation of Fe <2+> ions to Fe <3+> ions during the process to keep the concentration of the latter above the minimum pre-established value is carried out with an oxidizing agent added continuously to the bath in a controlled quantity based on to the value of the Redox potential which is measured continuously or periodically. The pickling bath is generally prepared with an initial quantity of oxidant in order to guarantee an adequate value of the Redox potential even in the start-up phase of the process, chosen on the basis of the type of steel to be pickled, the surface characteristics of the product (or semi-finished product) , the quantity and quality of the rolling or annealing scale.

The addition of oxidant during the process is substantially controlled as a function of the predetermined oxidation potential for the bath: this potential is thus maintained at the predetermined value. The stainless steel pickling processes in many cases provide for a final passivation treatment of the pickled material: this treatment can be carried out in a bath with a composition similar to that of pickling, however, brought to a higher Redox potential and in which HCl is advantageously replaced by Η3ΡΟ4. The baths in which class a), c) oxidants are used are those which are most suitable for this process.

Continuous insufflation of air

During the pickling operation, a continuous flow of air is maintained in the bath, in the ratio of at least 3 m <3> / m <3> of bath and per hour of processing. This volume of air introduced with an adequate and constant flow contributes to a good agitation of the bath, an important condition for an effective pickling as it continuously disturbs the liminal layer near the surface to be treated and thus allows it to be directly in contact with an always renewed pickling solution. To ensure an excellent mechanical stirring action and homogenization of the treatment liquid, it is advisable for the air to be blown from the lower part of the tank by means of perforated supply pipes or suitable blowing equipment. Instead of the insufflation of air, other stirring systems of the pickling solution can be adopted which ensure a perfect homogenization of the same and a continuous renewal of the solution in contact with the metal surface to be pickled: systems that create a strong fluid-dynamic effect in the liquid bath can be adopted. (ultrasound, forced circulation of the liquid), or the liquid is sent to the surface to be treated in the form of liquid jets or sprays.

Control of the Redox potential: as is well known, the behavior of stainless steel in acid mixtures is characterized by polarization curves which present, for different Redox potential values, phases of activity, passivity and transpassivity (see fig. 1).

Caption of figure 1

EO2EH2 equilibrium potentials of the development reactions of 0 and H

Ep critical passivation potential

Epe potential for complete passivation

Eo free corrosion potential or zero (external) current

EM equilibrium potential of the anodic dissolution reaction of the alloy

ET potential for transpassivation

However, the typical curve of fig.1 is valid for a steel of uniform composition and above all with a chromium content sufficient to give it passivity (Cr> 12%); the decrease in the chromium content modifies the polarization curve in the direction shown in fig. 2, reducing the passivity field, increasing the passivity current density and raising the critical passivation potential. Since a stainless steel, which is the object of the pickling process according to the invention, always has a more or less deep layer of decromized alloy, i.e. of chromium with respect to its basic composition, its polarization curve always shows the trend of fig. 3, in which the peak of the decromized alloy is evident, in a more or less defined way.

To ensure that the actual descaling and complete removal of the decromized alloy take place in the pickling phase. with restoration of maximum passivity on the surface, it is necessary to put the bath in conditions of potentiostatic control: it is a question of driving the working Redox potential so that, in the actual pickling phase, it remains in the range in which the speed is maximum of anodic dissolution of the decromized alloy with respect to that of the base alloy (dashed area in figure 3) · It is possible to pre-set this interval according to the type of steel ensuring, once the decronized alloy has been removed, the passivation of the base material.

During exercise, as the concentration of the divalent iron ion in the bath increases, the Redox potential of the bath tends to decrease, but the addition of the oxidant allows the values to be maintained at optimal levels, normally above 300 mV. In the procedures carried out, the value of 700 mV is never exceeded.

Wanting to obtain both the pickling and the subsequent passivation of the material in the same bath, the result is a Redox power of at least 350 mV.

In the case of particular processing upstream of the material and if a subsequent passivation stage in a separate bath is envisaged, the potential of the pickling bath can be kept at lower values, however not lower than 200 mV. The Redox potential of the pickling solution is measured with a platinum electrode and a reference electrode, for example of the calomel type or of the Ag / AgCl type.

With constant control of the potential it is therefore possible to guarantee not only a good pickling of the material, but also the subsequent passivation of the pickled material with excellent results. The industrial tests carried out have allowed, in fact, to obtain in any case shiny and bright surfaces, perfectly leveled, without any signs of any corrosive attack due for example to pitting phenomena, to "burning" of the material or to excess pickling action.

Contents of various additives in the pickling bath In the formation of the pickling bath according to the present invention, additives that are usual for this type of process, selected from the class of non-ionic surfactants, are used in the total amount of about .. g / l of bath. , having the function of wetting agents, emulsifiers, brighteners, and also inhibitors of acid attack. These additives, with mutual synergistic action, improve the pickling action by favoring it.

Particularly advantageous are additives consisting of perfluorinated anionic surfactants and also of non-ionic surfactants of the class of alkoxylated derivatives of alcohols.

The efficiency of the inhibitor action guarantees the protection of the base metal and drastically reduces the loss, thus being very effective especially for those discontinuous processes that require long pickling times (wire rods, tubes, bars). The additives present in the bath, and in particular the acid attack inhibitors, do not inhibit, however, the attack on the thermal oxides and. therefore, they do not in any way limit the pickling kinetics, as documented, for example, by the results of tests conducted on AISI 304 shot-blasted steel sheets.

Advantages of the procedure

Absence of sludge: the process according to the invention reduces to a minimum quantity or even prevents the formation of sludge and sludge, with consequent, evident, further economic savings.

This advantage is due among other things to an appropriate concentration of HCl during the process, as well as to the control of the concentration of ferrous ions, readily and adequately oxidized to ferric ions.

Unlike traditional baths, operating with nitric and hydrofluoric acid, the sludge and sludge produced, in much lower quantities as mentioned, are presented in the form of greenish, crumbly and incoherent pulp in the dry state, with negligible tendency to pack and 'agglomeration in large crystals and therefore easily removable.

Possibility of automatic control: the process has the possibility of being constantly kept under control with automatic equipment which, through analytical determinations (content of free acids, content of iron ions, Redox potential) continuously dose the quantities of pickling products and oxidant to be fed to ensure the correct operating parameters.

Versatility of the process: the process according to the invention is well suited to all existing industrial plants that treat stainless steel, requiring only modest adjustments. It is also suitable for the treatment of manufactured and semi-finished products of any type, from wire to wire rod, from strips to sheets and tubes, as the treatment parameters (temperature, times, concentrations) can be varied without absolutely compromising the results.

An emblematic example of this versatility is given by the possibility of combining the pickling according to the invention with a passivation treatment preferably carried out in a separate bathroom as mentioned above. The following examples are given purely by way of illustration of the application possibilities of the process according to the invention.

Example 1

Continuous strip sheets from cold rolling plants, made up of both 300 and 400 series steel, were treated according to a pickling process which includes the following steps:

a) electrolytic pickling treatment with H2SO4 for 1 'at 60 ° + 70 ° C, carried out in the first tank;

b) pickling treatment with bath according to the invention, carried out in tank 2 with a treatment time of 2 '; the composition and characteristics of the pickling solutions are indicated in the following tables.

The useful capacity of the bathroom of the 2nd basin is 17 m <3>.

During the process, air is continuously forced at a flow rate of 10 m <3> / m <3> / h and both the oxidant consisting of H2O2 in the form of an aqueous solution at 35% P and the others are continuously fed into the bath ingredients (HF and HCl) in order to maintain the concentrations and the Redox potential at the predetermined values.

In all cases the pickled material passed to a passivating treatment carried out in a 3rd tank containing:

HF 5 g / l

Fe <3+> 2 g / l

H3PO4 25 g / l

And Redox mV 500 approx.

The value of the Redox potential was maintained at the predetermined value with periodic additions of H2O2. The bath temperature was below 30 ° C, the treatment duration was 90 ".

The surface appearance of the sheets at the end of the process was always shiny and bright.

Also in this case on no material there was evidence of over-pickling or surface corrosive attack.

Claims (6)

CLAIMS 1. Process for stainless steel pickling consisting in putting the material to be treated in contact with a bath maintained at a temperature between 30 ° and 70 ° C, preferably between 45 ° and 55 ° C and having the following composition a) HCl from 25 to 120 g / l b) Fe <3+> at least 15 g / l preferably at least 30 g / l c) HF from 5 to 50 g / l d) emulsifying additives, wetting agents, brighteners and acid attack inhibitors: about 1 g / l in total: in said bath maintained under agitation by a continuous flow of air or with another equivalent agitation system being fed continuously, or periodically: quantities of ingredients a), c), d) suitable for maintaining their concentration in the bath at optimal levels and the pH of the bath itself at values <2.5 and also an oxidizing agent in a suitable quantity to maintain the Redox potential of the bath at values not less than 200 mV, said oxidizing agent being selected from the following classes of compounds: a) H2O2, peroxygenated acids such as persulfuric acid, its salts b) oxygenated chlorine acids: chlorine and chloric in the form of their alkaline salts such as Na ClO2, Na ClO3 c) alkaline permanganates said oxidants being able to be introduced into the bath as such or in the form of an aqueous solution. 2. Process according to claim 1, wherein the treatment bath is adjusted to an oxidation potential value of at least 350 mV. thus obtaining an adequate passivation of the pickled material. 3- Process according to one of the preceding claims, in which a continuous flow of air equal to at least 3m <3> / h per bath is introduced into the pickling bath by means of a diffusing device for diffusing the flow itself in the liquid mass. 4. Process according to claim 1 in which the Fe <3+> ions are introduced into the initial bath in the form of ferric sulphate. 5. Process according to claim 1 in which, as a function of wetting agents, emulsifiers, brighteners and acid attack inhibitors, a surfactant belonging to the following categories is used: non-ionic surfactant of the class of alkoxylated derivatives of alcohols having atoms C> 6, surfactant perfluorinated amionic. 6. Process according to claim 1 carried out in combination with a preliminary treatment of partial removal of the oxides by means of a known mechanical process.